Hormone replacement therapy may be even more likely to cause breast cancer than we thought. But how much bigger is the risk, and is it so high that no one should take it?

Why do women take HRT?

The menopause is natural, but can be painful. Between the ages of 45 and 55, women largely stop making the hormones oestrogen and progesterone, and this can cause hot flushes, insomnia, depression and loss of libido, usually for around four years.

Since the 1940s, doctors have been giving women oestrogen to reverse these symptoms, but by 1975 it had become clear that this caused higher rates of uterine cancer. Combination HRT also includes the hormone progesterone, to protect against this, and in 2003, 15 per cent of women in the UK between the ages of 45 and 68 were taking this treatment.

When was the breast cancer link discovered?

Breast cancer was first shown to be associated with combined HRT treatment in 2003, when a large US trial found that the risk of developing breast cancer was 1.26 times higher for women taking HRT than those who weren’t. Then a UK study found a doubled risk of breast cancer in women taking HRT, and calculated that this led to 15,000 extra cases of the disease between 1991 and 2005.

These findings caused uptake to plummet, and by 2013, only 5 per cent of women in the UK between the ages of 45 and 69 were taking HRT. However, more women may now be choosing to take the treatment since researchers started finding it had other benefits.

How big is the risk?

The UK National Health Service advises that the therapy is likely to cause five more cases of breast cancer than the 22 normally expected per 1000 post-menopausal women, plus one extra death from ovarian cancer. The risk returns to normal five years after a woman stops taking HRT.

But Michael Jones at the Institute of Cancer Research in London and his team think this is an underestimate, due to incomplete data in the earlier studies. Using a recent, more detailed study of 114,000 British women, they calculated that women who take HRT for five years have a 2.74 times higher risk of developing breast cancer than women who never take it. This may mean the number of extra cases per 1000 women caused by HRT could be as high as 38.

Will women still take it?

It depends on the various risks and benefits. In younger menopausal women, HRT’s preventative effects on coronary heart disease, bone fracture and colon cancer may outweigh the increased incidences of breast cancer, stroke and pulmonary embolism associated with the treatment. But in older women, these risks may outweigh the benefits. “For each woman there will be a balance between risks and benefits,” says Jones.